An INVAR.RTM. or iron nickel alloy having the composition of Fe-36 wt % Ni has been known as an alloy having a low thermal expansion property. Such an alloy is used for precision parts, for example. In order to increase the transmission capacity of an aluminum cable steel reinforced (ACSR) as overhead conductor cable, a method of reducing cable slack of the conductor cable caused by an increase in temperature during power transmission, has been studied. One method has been known in which an alloy wire having a low thermal expansion property is used as a steel core to reduce cable slack. An INVAR.RTM. or iron nickel alloy wire such as disclosed in Japanese Patent Laying-Open No. 55-119156 has been developed as an alloy wire having a low thermal expansion property.
The alloy wire developed in accordance with Japanese Patent Laying-Open No. 55-119156 is a hard material and exhibits a tensile strength of 120 kg/mm.sup.2. However, it exhibits a low toughness property stability such as turns of twisting after it has been finally subjected to zinc or zinc alloy plating, thereby reducing the production yield of conductor cables. Zinc alloy plating or the like is applied to improve the corrosion resistance of the conductor cable. However, an intermetallic compound formed at the interface with the plating tends to lower the twisting property of the alloy wire.
Therefore, it is an object of the present invention to improve the toughness of a conventionally used INVAR.RTM. or iron nickel alloy wire having high strength, and, more particularly, to improve the twisting property or twisting ability of the wire in its finished wire size.